by Ann Oldenburg, USA TODAY

by Ann Oldenburg, USA TODAY

After a crazy week in London, the second of two Justin Bieber concerts scheduled for this week in Portugal has been canceled, promoters in Lisbon said Monday.

It was not immediately clear why the show was nixed. No word on if it had anything to do with the pop star's backstage collapse in London last week, which prompted Beiber to take 20-minute break to receive oxygen and then to stay overnight in a hospital.

Bieber is set to play tonight at Pavilhao Atlantico.

"Due to unforeseen circumstances, Justin Bieber was forced to cancel the second performance in Portugal, March 12," a statement said on the promoters' website.

"The Canadian singer is eager to play for the Portuguese fans on March 11," it added. Ticket holders for the canceled show are entitled to a refund if they claim it within a month.

Bieber has not mentioned the cancellation on Twitter (his latest tweet is: "#sundayfunday"), nor has his manager Scooter Braun, who is at at the SXSW festival in Austin.

Showbiz411 columnist Roger Friedman reports the reason for the cancellation is "poor ticket sales." And The Christian Post cites a Portuguese newspaper as suggesting "that although the first date on Monday had been sold out, the Tuesday concert had an insufficient sale of tickets."